The obese (ob) gene product, leptin, is an important circulating signal for the regulation of body weight (Zhang Y. et al. (1994) Nature 372: 425-432). Mice homozygous for a nonfunctional ob gene become morbidly obese and diabetic, due to overeating and increased metabolic efficiency. In 1995, Tartaglia L. A. et al. (Cell 83: 1263-1271) described a high affinity receptor for murine leptin (OB-R). Evidence suggests that the weight-reducing effects of leptin may be mediated by signal transduction through OB-R in the hypothalamus (Lee G. H. et al. (1996) Nature 379: 632-635). In addition to reducing appetite, leptin has been found to ablate body fat in rats (Chen G. et al. (1996) Proc. Natl. Acad. Sci. 93: 14795-14799). The injection of a leptin-expressing adenoviral vector resulted in reduced body fat relative to rats that did not receive the leptin-expressing vector, but which ate an equivalent amount of food.
Regulation in the expression of splice variants can have the important role in the activity of signal transduction molecules and has been implicated in the pathogenesis of several diseases (Khachigian L. M. et al. (1992) Pathology 24: 280-290). For example, mutations that create new splice variants of the sulfonylurea receptor gene segregate with familial persistent hyperinsulinemic hypoglycemia (Thomas P. M. et al. (1995) Science 268: 426-429).
At least nine alternatively spliced forms of mouse OB-R have been described (Lee et al., supra). A splice variant, B219, is expressed in the mouse yolk sac, early fetal liver, enriched hematopoietic stem cells, a variety of lympho-hematopoietic cells lines, and in adult reproductive organs and may be directly involved in hematopoiesis and reproduction (Cioffi J. A. et al. (1996) Nature Medicine 2: 585-589). Further evidence of a role for leptin in hematopoiesis was recently reported by Gainsford T. et al., who found that leptin enhances cytokine production and phagocytosis of parasites by murine peritoneal macrophages (1996, Proc. Natl. Acad. Sci. 93: 14564-14568). Additional support for a leptin role in reproduction comes from Chehab F. F. et al, who report that treatment with leptin corrects a sterility defect in ob/ob female mice (1996, Nature Genet. 12: 318-320). The researchers showed that leptin brings on fertility by restoring necessary hypothalamic and pituitary hormone levels rather than by fat reduction.
Recently, leptin has been implicated in the induction of puberty in both mice and humans. Female mice treated with regular injections of leptin reach sexual maturity 37 days after birth, whereas untreated mice show signs of sexual maturity at about 40 days after birth (Chehab F. et al., (1997) Science 275: 88-90). While in humans, leptin levels increase sharply at the same time that testosterone levels increase in boys undergoing puberty (Flier J., unpublished).
OB-R mutations that create alternatively spliced transcripts are responsible for the severely obese and diabetic phenotype of db/db mice (Chen H. et al. (1996) Cell 84: 491-495) and of fa/fa Zucker rats (Chua S. C. et al. (1996) Science 271: 994-996). Based on synteni between human and mouse chromosomes, the human version of OB-R is likely to map to human chromosome 1p31 (Lee et al., supra).
Genome sequencing efforts in Caenorhabditis elegans and Saccharomyces cerevisiae have revealed putative open reading frames (ORFs) C30B5.2 and YJR044c, respectively (Wilson R. et al., (1994) Nature 368: 32-38; Huang M. E. et al. (1995) Yeast 11: 775-781). YJR044c and C30B5.2 are 27% identical and 71% similar in amino acid sequence and share a similar pattern of hypdrophobicity. YJR044c has been characterized as a putative membrane associated protein (Wilson et al, supra). The C30B5.2 amino acid sequence has a consensus pattern (CCxxHxxC) for phospholipase A2, a family of enzymes that release fatty acids from the second carbon group of glycerol.
The activity of many signal transduction molecules, such as the leptin receptor, is thought to be regulated by the expression of splice variants of the molecule. A new leptin receptor gene-related protein could provide the basis for diagnosis and treatment of cancer, and disorders in energy metabolism, reproduction, connective tissues, and development.